Proper ventilation and regulation of humidity are essential for maintaining healthy and comfortable air quality indoors. However, these two factors can be in conflict in certain situations. For example, when ventilation rates are increased to improve indoor air quality, humidity can soar to levels that are uncomfortable or even unhealthy. Nearly all residential heating, ventilation and air conditioning (HVAC) systems are capable of regulating air temperature within acceptable ranges. However, few systems are able to effectively regulate air humidity.
People living in the eastern portion of the United States are familiar with the problem of less than adequate humidity control. A rainy summer night with temperatures in the range of upper 60s to low 70s can have a humidity ratio above 0.015 lb/lb (dewpoint above 68° F.). Since the sun is down and the air temperature is moderate, the cooling load on the house is almost zero. If the air conditioner does not run, the absolute humidity within the house will equal or exceed that of the outdoors. For a 75° F. indoor temperature, the relative humidity will be at least 80%—a level that is not only uncomfortable, but exceeds the 70% threshold at which mold and mildew proliferate.
Conventional HVAC equipment under such conditions is limited in its ability to restore comfortable air quality. All conventional systems dehumidify by cooling air below its dewpoint. A conventional vapor compression dehumidifier operates by cooling the air to condense the water vapor, and thereafter re-heating the air. However, this process is generally inefficient.
Desiccants provide a very efficient means to control indoor humidity independent of temperature. The concepts described herein integrate desiccant technology with a vapor-compression air conditioner to produce a system that yields an enhanced dehumidifier exhibiting higher efficiency.
Attempts have been made to develop vapor-compression air conditioners that directly coupled a liquid desiccant to both the evaporator and condenser of the air conditioner. The earliest work was done by John Howell and John Peterson at the University of Texas. The concept involved spraying desiccant directly onto the air conditioner's evaporator and condenser. The process air stream that flows through the evaporator is simultaneously cooled and dehumidified as the desiccant absorbs water vapor from the air. The cooling air that flows through the condenser, in addition to carrying away the heat rejected by the air conditioner, regenerates the desiccant by carrying away water desorbed by the warm desiccant.
Although Howell and Peterson modeled the performance of a liquid-desiccant vapor-compression air conditioner (LDVCAC) that used lithium chloride, the prototype that they built and tested used ethylene glycol. Unfortunately, the use of glycol as a desiccant was impractical. All glycols have a finite vapor pressure. In both the evaporator and the condenser, glycol will evaporate into the air streams, thus undesirably requiring periodic recharging of the system.
More recently, the Drykor Corporation of Israel introduced several models of liquid-desiccant vapor-compression air conditioners (LDVCAC) based on the teachings of U.S. Published patent application Ser. No. 2002/0116935. The Drykor technology uses lithium chloride as the liquid desiccant. This is an improvement over the Howell and Peterson work since solutions of all ionic salts including lithium chloride do not “evaporate” the salt, i.e., the vapor pressure of an ionic salt is essentially zero.
In the Drykor system, the liquid desiccant is first cooled in the evaporator in the form of a refrigerant-to-desiccant heat exchanger, and then the cool desiccant is delivered to a porous bed of contact media where the process air is dried and cooled. Similarly, the desiccant is regenerated by first heating it in the condenser in the form of a second refrigerant-to-desiccant heat exchanger and then flowing the warm desiccant over a porous bed of contact media where a stream of ambient air is flowing therethrough.
The American Genius Corporation (AGC) is marketing a liquid desiccant air conditioner that functions similarly to the Drykor unit. The AGC system uses a mixture of lithium chloride and lithium bromide as the liquid desiccant.
In one important way, the LDVCAC of Howell and Peterson is superior to those of both Drykor and AGC in that the Howell and Peterson system uses the evaporator and condenser of the vapor-compression air conditioner as the contact surface for mass and heat exchange between the desiccant and the air streams, whereas the other two systems either heat or cool the desiccant and then, in separate sections bring the desiccant in contact with the air streams. The LDVCACs of Drykor and AGC therefore introduce additional temperature drops that degrade the efficiency of the air conditioners.
The LDVCAC of Howell and Peterson, however, cannot be easily used with aqueous solutions of either lithium chloride or lithium bromide because these solutions are very corrosive to the metals that are commonly used to make evaporators and condensers. While the evaporator and condenser can be made from an expensive alloy that resists corrosion, the resulting air conditioner would be too expensive to sell in the broad HVAC market. Howell and Peterson suggested that corrosion-resistant metallic tubes with plastic or ceramic-coated fins may be a compromise surface for combined heat and mass transfer. However, these approaches of protecting the evaporator and condenser from corrosion have important limitations: plastics have a low surface energy and so are not easily wetted by liquids; and ceramics are very difficult to apply in the thin pin-hole-free coatings needed in this application.
All LDVCACs must also prevent droplets of desiccant from being entrained by the air that flows through the dehumidifying and the regenerating sections of the air conditioner. While it is possible to add a droplet filter or demister at the air exits from both the dehumidifying and regenerating sections of the LDVCAC so that droplets do not escape from the system, this approach will create large maintenance requirements associated with keeping the filters unblocked by liquid, and increase the pressure drop that must be overcome by the system's fans.
U.S. Pat. Nos. 5,351,497 and 6,745,826 teach that desiccant droplets can be suppressed in a mass and heat exchanger by flowing very low rates of desiccant onto the surfaces of the mass and heat exchanger, and preparing the surfaces so that the low flow of desiccant still provides uniform coverage. This approach to suppressing droplets cannot be used in the LDVCACs proposed by Howell-Peterson, Drykor or AGC. As previously described, in the Drykor and AGC systems the desiccant is first heated or cooled in a refrigerant-to-desiccant heat exchanger and then the desiccant is brought in contact with air in a bed of porous contact media. The bed is adiabatic (i.e. the bed does not exchange thermal energy with the desiccant). The flow rate of desiccant, therefore, must be high enough to prevent the temperature of the desiccant from either decreasing too much (in the regenerating section where the desorption of water is endothermic) or increasing too much (in the dehumidifying section where the absorption of water is exothermic). This prevents the use of Lowenstein's low-flow approach to suppressing droplets.
In the Howell-Peterson LDVCAC, the contact surface on which the desiccant and air exchange heat and mass is either the surface of the evaporator or the condenser. Thus, if these heat exchangers have metallic fins, the desiccant will be continually cooled or heated as it interacts with the air. However, the Howell-Peterson LDVCAC does not readily achieve uniform distribution of the desiccant on the surfaces of the evaporator and condenser. As noted earlier, Howell and Peterson propose that the evaporator and condenser can be coated with plastic or ceramic to protect them from a corrosive desiccant. However, these coatings do not enhance and may deter the spreading of the desiccant over the external surfaces of the heat exchangers. Furthermore, Lowenstein's low-flow approach to suppressing droplets would be difficult to implement with plain plastic surfaces.
Howell and Peterson's suggestion that corrosion-resistant metallic tubes be used with plastic fins is also disadvantageous because of the poor thermal conductivity of plastics. Although a plastic fin can be used to provide contact between the liquid desiccant and the air that flows over the fin, the fin will not effectively heat or cool the desiccant. It is essential in a heat and mass exchanger that the liquid that flows on the fins periodically comes into close thermal contact with the metallic tubes. We have observed that the most common configuration for finned-tube HVAC heat exchangers (e.g. FIG. 3 of U.S. Pat. No. 4,984,434), in which the tubes pass through holes in the fins, will not effectively heat or cool the desiccant if the fins are plastic, even if the surface of the fins are treated so that uniform films of desiccant are created. This is because the plastic fins are poor thermal conductors and they provide a path for the desiccant to bypass the tube i.e., the liquid desiccant can flow on a fin from the top of the evaporator/condenser to the bottom without ever coming in thermal contact with a metallic tube.
The evaporator and the condenser of a LDVCAC are heat and mass exchangers whereby in the form of an evaporator both thermal energy (heat) and water vapor (mass) are absorbed from an air stream, and whereby in the form of a condenser both heat and mass are added to an air stream. Many processes in industry rely on mass and heat exchangers, and the invention can be used to both lower the cost and improve the efficiency of some of these processes. Examples of processes that may benefit from the invention are: (1) evaporative condensers for air conditioners and refrigeration systems, (2) gas scrubbers used in emission control systems and gas purification systems, (3) desalination plants, (4) driers, distillers and concentrators where water or other volatile species are removed from a less-volatile liquid, and (5) absorption chillers.
The heat and mass exchangers for the preceding processes are commonly configured as an array of tubes that can be oriented vertically or horizontally. If the process is endothermic, as would be the case for most evaporation, distillation or desorption processes, the tubes are heated internally through a fluid or condensing vapor such as steam. The second fluid that is to be evaporated or that contains the volatile specie that is to be desorbed flows as a film over the outside of the tubes.
In at least one configuration of a heat and mass exchanger, which is described by Goel and Goswami in the Fall 2004 Newsletter of the ASME Solar Energy Division, the external surface of the tubes is enhanced with a screen, mesh or fabric. For a vertical column of spaced-apart horizontal tubes, the screen, mesh or fabric is interlaced with the tubes so that it alternately contacts the left and right sides of the tubes at a limited region of contact. As an absorbing fluid flows downward in the screen, mesh or fabric, it contacts each tube in the column in this limited region of contact, but the liquid is not forced to flow around the tube.
Accordingly, there is a need for a heat and mass exchanger for use in a thermodynamic device that is designed to overcome the limitations described above. There is a need for a heat and mass exchanger that can carry a liquid on the surface of the exchanger that either absorbs, desorbs, evaporates or condenses one or more gaseous species from or to a surrounding gas such as a process air stream, while maintaining the temperature of the liquid at a desired level to improve the efficiency of the heat and mass exchange. There is a further need for a heat and mass exchanger compatible with corrosive liquids such as liquid desiccants, and which is capable of suppressing droplet formation of the liquid, while maintaining both elevated levels of efficiency and ease of maintenance.